The zebra finch song system consists of sexually dimorphic nuclei. Early estrogen treatment masculinizes telencephalic song nuclei in genetic females, but blocking estrogen does not demasculinize these nuclei in genetic males. Studies have suggested that the existence of a genetic factor intrinsic to the brain underlies the masculine pattern of song system organization. Recently, a sex difference has been seen in transcriptional levels of 17 alpha hydroxylase (CYP 17), the enzyme that makes androgen, in the telencephalon of developing zebra finches. It is hypothesized that GYP 17 expression extends through the steroid sensitive period of song system development, and is distributed near the song nuclei. This will be investigated using in situ hybridization. It is hypothesized that the different levels of CYP17 mRNA reflect sex differences in protein-mediated transcriptional regulation. DNA sequence from the promoter region of the CYP 17 gene will be determined by screening a cDNA library, performing 5? RACE, and screening a zebra finch genomic library. This represents a first step in identifying the factors that differentially regulate CYP 17 transcription in the telencephalon of developing male and female zebra finch. They will also indicate potential regulatory mechanisms for androgen synthesis in other species, including humans.